The present invention relates generally to a truck transmission and, more particularly, to a transmission having at least three overdrive configurations.
Heavy duty class 8 manual truck transmissions are geared to match the engine speed with the axle carrier ratio and wheel size. Typically the top transmission gear ratio is matched in the vehicle to give a specific narrow band engine speed at top vehicle speed. Under heavy weighted vehicle applications, the ratio between steps may be too great causing the engine speed to fall out of the narrow fuel efficient engine band. The industry standard for heavy duty truck manual transmissions is to create a split ratio in between ratios to maintain ideal engine speed. Historically, this split has created an overdrive feature.
Overdrive refers to a combination of gearing inside a transmission which results in the output speed being greater than the input speed. Overdrive allows the engine to operate at a lower revolutions per minute (RPMs) for a given road speed. This allows the vehicle to achieve better fuel efficiency and often quieter operation on the highway.
Heavy duty truck transmissions are presently often made with eighteen speeds. These are achieved using a standard nine speed transmission and adding a splitter to the rear of the transmission, thereby doubling the amount of speeds. Normally this splitter is geared as an under drive ratio, limiting the number of overdrive ratios available in the transmission.
More than two overdrive ratios may also be created by using multiple overdrive gears, as discussed in “The Trucker's Report” (http://www.thetruckersreport.com/truckingindustryforum/trucks-eighteen-wheelers/15366-old-gears-transmission-info-of-yesteryear.html).
Eaton Corp. Transmission Model RTOO-1463 has a main gear box and splitter that each have a gear configured with an overdrive ratio which permits three total overdrive ratios. These three ratio configurations include the main gear box in direct drive and the splitter in overdrive, the splitter in direct drive and the main gear box in overdrive, and both the splitter and the main gear box in overdrive. The RTOO-14613 contains a splitter section that is located between the range section and the output shaft. FIG. 1 schematically shows the RTOO-14163 transmission 1 in which one of the splitter gears sets 2 is disposed behind the range section 3, with the range section between the splitter section and the main box section 4, in a manner such that the transmission requires two compound main shafts 5, 6 connected together with a small pin (not shown) that runs through the center of both shafts. When the transmission is in the fastest overdrive ratio (the rear sliding clutch 7 is shifted to the right in FIG. 1 and the range synchronization clutch 8 is shifted to the left), there is a rotating relative speed difference between these two compound main shafts 5, 6 that are butted up against one another. This could lead to a friction weld and seizing of these two shafts 5, 6 together. If this were to occur the compound main shaft would be in two gears at the same time and a counter shaft could fail. This design requires operators to avoid range shifting with the splitter in overdrive. If a range shift is made with the splitter in overdrive, nothing happens because the two compound main shafts are disconnected. When one then shifts the splitter to a direct drive configuration, there would be too much of a ratio step for the unsynchronized clutch to engage and noisy ratcheting is likely to occur.
Because of the two compound main shaft design, it is necessary to synchronize the main shaft through the main box section 4 with the main shaft through the range section 3, and the main shaft through the range section with the main shaft through the splitter section 2. In addition to the potential problems noted above, the need for synchronization between the double compound main shafts complicates desirable practices such as progressive shifting because the splitter change must synchronize the entire compound section rotating inertia plus the front main box shaft inertia. Further, the design of this transmission limits it to 13 gears.
It is generally desirable to have a variety of overdrive options associated with an engine to improve fuel economy. It is also desirable to facilitate progressive shifting by minimizing the need for synchronization. It is also desirable to reduce torque downstream to the axle carriers. It is further desirable to reduce drive shaft torque allowing for the use of standard drive shafts and higher reduction axle carriers.
According to an aspect of the present invention, An overdrive transmission, comprising a main box section connectable to an input shaft, an auxiliary section connected to the main box section and connectable to an output shaft, the auxiliary section further comprising a splitter section, and a range section. The transmission is adapted to be configured in a plurality of gear combinations, at least three of the gear combinations are configured for overdrive, and the main box section is connected directly to the splitter section.
According to another aspect of the present invention, a method of making an overdrive transmission comprises connecting a main box to an input shaft, connecting an auxiliary section to the main box section, the auxiliary section comprising a splitter section and a range section, and connecting the auxiliary section an output shaft. The transmission is adapted to be configured in a plurality of gear combinations, at least three of the gear combinations are configured for overdrive, and the main box section is connected directly to the splitter section.